Why do dogs rub up against things that smell bad?

L'Eau Pour Chien

Why do dogs rub up against things that smell bad?

I'm no perfume connoisseur, but somewhere along the way, in a cheap strip mall in Brussels, my nose found a pleasurable, enigmatic scent called Eau des Baux, by the popular French chain L'Occitane. The company describes it as a "sensual and mysterious blend of cypress and incense." How fancy. I've always wanted to be sensual and mysterious, in fact, but, sadly, nature failed to endow me with either this rich blend of natural secretions or the corresponding personality attributes. So I shelled out some 34 euros, and prior to my book launch in Ohio a few months later, I entertained this olfactory illusion with a few liberal spritzes on my upper half. I must say, while I didn't feel especially erotic that night, I did smell delicious. My dog, Gulliver, would agree, since on getting a good whiff of me, he performed a shameless act of vertical scent-rubbing by grinding his cheeks and sides vigorously against my cologne-covered body.

The curious thing about this was that, earlier that same day—and in fact I'd just gotten through bathing him to wash away the mealy intestinal bits stuck in his fur—Gulliver had done the very same thing with the week-old, rotting carcass of an albino squirrel at the park. Most men might second-guess their sillage de parfum given these facts, since heaven knows we're not all turned on by a trail of festering squirrel juice, but I still liked it, myself. In any event, all of this had me wondering why it is that dogs indulge in such scent-rubbing behavior, and in response to such diverse cues as Eau des Baux and Eau de Squirrel Innards. (Note the distinction between scent-marking, which transfers the animal's signature scent to its surroundings, and scent-rubbing, which transfers environmental scents to the animal's body.) L'Occitane wasn't in business when domesticated dog brains were evolving, of course, but perhaps the general behavior has some adaptive function.

As it happens, a team of animal behaviorists led by Jenny Ryon at Dalhousie University had explored this very question of canines' penchant for perfume in 1986. Their study wasn't with dogs—to the best of my knowledge, no controlled studies on scent-rubbing with domesticated dogs have ever been done—but with a very close relative instead: the wolf (Canis lupus). The authors note that scent-rubbing is an unconditioned response in wolves, which means that it's an instinct, more or less, and they submit that wolves have been observed to luxuriate among the pungent, nostril-pinching stews of everything from detached body parts to insect repellent to rotten fruit and cigar ashes. That's quite a wide array of stinky things, some of which did not exist through most of evolutionary history.

Ryon and her coauthors' objective, however, was to conduct a controlled test of the idea that wolves will grind up on unfamiliar, strong-smelling objects as a way to obtain chemosensory information about them. This is a relatively easy hypothesis to test, because if it's correct, then simple repeated exposure to the same stimulus should lead to a decrease in rubbing. Fortunately, the authors had a few packs of wolves at their disposal in their university labs, wolves whose normal diets consisted of "raw chicken necks and backs and whole or partial carcasses of road-killed deer." (We can only assume there was no shortage of the latter on the Madonna-blaring highways of 1980s Nova Scotia.)

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Here's how the experiment worked. The researchers selected four pairs of "odiferous substances" and coated 15 cm circles of clean, demarcated ground area with these substances using a sterile wooden tongue depressor. The carnivore feces pair included the dung of black bears and cougars; herbivore feces consisted of the waste products of Aoudad sheep and the Sable Island Horse; the food class sampled albacore tuna oil and commercially produced salt pork; and, finally, the manufactured category was represented by heavy-duty motor oil and a now-discontinued Max Factor perfume called "Geminesse," which, just to be clear, was redolent of a leather-floral mix atop a musky, amber base.

The wolves were released into the pen one by one, and observed "from an automobile inside their enclosure"—there's no telling how insane Geminesse might make these beasts, after all. In contradiction to their hypothesis, explain the authors, the wolves showed a marked propensity to rub on some substances and not others, with manufactured odors eliciting the strongest response. All individuals that rubbed did so most often on the perfume. Interestingly, just as Gulliver did with me, some of the wolves waiting their turn performed the same type of "vertical scent-rubbing" on the perfumed bodies of those that had just returned from the pen. Carnivore feces had them rolling around on the malodorous dirt too, but none of the other stimuli, herbivore dung and food, did the trick.

The authors argue that it's not novelty or even pungency per se that elicits scent-rubbing; some especially smelly substances, such as the sheep shit and the tuna oil, went unmolested. Rather, "[o]ur wolves showed a negative correlation between substances rubbed and substances tasted. Manufactured and carnivore odors were most frequently rubbed but rarely tasted, whereas the reverse applied to food and herbivore odors." In other words, the wolves didn't rub against the sheep shit and tuna oil; they licked or even ate it. These findings also cast considerable doubt on another theory, not mentioned by these authors but circulating in dog-behavior crowds for some time, which is that dogs scent-roll to "camouflage" their own odors. This evolutionary theory sounds good in principle: It would have helped ancestral canines thwart detection by prey animals that bolted whenever they got wind of wolf in the air. Yet, if it were true, then we wouldn't expect the wolves in the foregoing study to behave as they did, since it's hard to imagine that smelling like a bear or cougar is much better than smelling like a wolf for this purpose, and it's certainly less effective than masking predator odor with that of a harmless ungulate.

Ryon and her colleagues concede that it's difficult to make sense of these data, since, after all, "if wolves are differentially sensitive to certain odors, they should be more sensitive to substances of biological relevance such as herbivore feces." But they speculate that the animals' rolling around in motor oil and perfume may reflect a mnemonic strategy, wherein the wolves learn about odors that signal potentially important, deviant changes in their environment. These changes may be evolutionarily relevant (competitive predators) or just really weird (a leather-floral mix) and therefore of uncertain adaptive importance.

More recently, another group of animal behaviorists got back into this dirty business, but traipsed down the taxonomic line to spotted hyenas—a scent-rubbing species that is more distantly related to domestic dogs. For a 2002 article in the Journal of Comparative Psychology, Christine Drea and her colleagues were keen to understand the specific function of the behavior, at least as it occurs in spotted hyenas. The authors hypothesize that rolling around in rot serves not only a memory-related function, but also an important social function for hyenas. Whatever information is contained in the odor would be conveyed from one hyena to another, their theory goes, in the animals' characteristic greeting display—a ritualistic activity that sounds remarkably similar to my own reunions with my partner after some time apart:

During a greet, participants first sniff each other's mouth, head, and back, and then, while displaying erections, stand head to tail and mutually inspect each other's genitals, flanks, and bulging anal pouch. … In nature, a hyena that rolls in an odiferous substance ultimately brings that odor back to the clan. If rolling serves a social function, its consequences are likely to be evident when hyenas reunite.

To determine the social function of scent-rubbing in this species, an investigator surreptitiously squirted one of two different odors (either carrion, which reliably elicits spontaneous rolling among wild hyenas, or camphor, which doesn't) upon the necks, head and shoulder regions of individual hyenas while petting them. Then the "odor-wearer" animal was released into the yard and its behavioral interactions with another hyena, the "odor-perceiver," were observed.

Why do dogs roll around in smelly stuff?

As predicted, hyenas perfumed with carrion received significantly more attention than did the camphor-dappled or those not wearing any odor. Donning eau de carrion led to heightened and positive social interactions—lots of anal bulging—between the two, regardless of the social status of the odor-wearer. Termination of the ceremony also ended peacefully with the carrion odor, unlike some of the other greetings, which met with aggression. It's probably wise to be on friendly terms with the guy who can lead you to the meat. (Or the gal, but female spotted hyenas have faux male genitalia, so gender-equality grammar may be overkill here.) "As olfactory cues and social sniffing are frequently used to unify the clan or to rally hyenas into group activity," Drea and her co-authors conclude, "rolling may ultimately promote social cohesion."

It's not clear how much of these wolf and hyena findings you can extrapolate to your own dog's behavior next time she rolls around in rotting duck gizzards or badger vomit. But cut your bitch some slack and don't make her feel ashamed about it: The putrid smell of microbial fermentation is her own personal Geminesse.

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